For videotext (BTX) it is known to transmit a so-called FSK (frequency shift keyed) signal over the telephone network. This signal is a sinusoidal oscillation whose frequency is switched between two values corresponding to a digital signal to be transmitted. The one frequency is here 1,300 Hz and corresponds to the logic value HIGH or 1 in the digital signal. The second frequency is 2,100 Hz and corresponds to the logic value LOW or 0 in the digital signal.
Such an FSK signal is an analog signal from which a digital signal is generated in a decoder. The digital signal then serves, for example, to generate character signals in a character generator which signals, after storage of the FSK signals transmitted for a longer period of time, constitute a page of text on the screen of a television receiver.
It is known to decode the FSK signal by subjecting the signal to a PLL (phase locked loop) phase control loop whose voltage controlled oscillator (VCO) is regulated to the frequency of the respective FSK signal. For this regulation it is necessary that, if the FSK signal changes its frequency, the regulating voltage generated in the phase comparison stage of the PLL circuit and transmitted through a lowpass filter also changes. This changing voltage thus contains the information contained in the FSK signal. It is fed as signal voltage through a further lowpass filter to a circuit having a threshold value characteristic, e.g. a trigger element which generates at its output a digital signal corresponding to the frequency keying of the FSK signal. Such an FSK decoder including a PLL circuit is described in detail in the in-house publication entitled "FSK Demodulator/Tone Decoder", No. XR-2211, of EXAR INTEGRATED SYSTEMS, INC., October 1976, pages 1-6.
The values of the above-mentioned signal voltage at the input of the trigger element depend on various factors, e.g. the frequencies of the FSK signal, the natural frequency of the oscillator of the PLL circuit and the tolerances of the individual components. In practice, these parameters are subject to fluctuations so that the digital signal at the output of the above-mentioned trigger element may be falsified. A signal section of the signal which corresponds, e.g., to a signal section at a certain frequency in the FSK signal, may then be shorted, for example. This produces so-called isochronous distortions during decoding of the digital signal.
To avoid such isochronous distortions, it is necessary to precisely match the PLL decoder. This matching is necessary during manufacture of the decoder. If the stated parameters change at a later time, further matching may also be necessary during operation.